1. Field of the Invention
The invention relates to a moving-target indicator system (MTI) utilizing adaptive-loop concellers for cancelling clutter. More particularly, the adaptive-loop cancellers utilize multiple memories to store the developed weight W for selected range intervals on a pulse-to-pulse basis.
2. Description of the Prior Art
In MTI systems the received radar-return pulses include desired target reflections and undesired clutter from the sea surface, slow-moving rain squalls, chaff, etc. The radar clutter is correlated from pulse-to-pulse while the target reflections are moving quickly and have a phase change from pulse-to-pulse, thus being uncorrelated. An MTI system cancels all correlated signals on a pulse-to-pulse basis, thus cancelling clutter, and passes uncorrelated signals, thus passing target reflections.
The use of adaptive loop cancellers to cancel correlated signals in moving target indicator systems is known. One type of adaptive-closed-loop canceller (FIGS. 1, 2), disclosed in U.S. Pat. No. 3,978,483, uses a pair of adaptive loops which separately compute the I and Q components of the complex weight and use D.C. weights in each loop. A second type of adaptive loop canceller, which is the subject of copending U.S. Pat. application Ser. No. 818,180, now Pat. No. 4,086,592, (assigned to the Navy), not herein considered to be prior art, is a digital open loop canceller which computes the optimum weight digitally with an open loop.
A problem common to these loops is that the averaging time taken to build up the weight W for each of the different types of loop must be relatively fast. This is because each returned radar pulse covers a time span during which radar reflections from different sources of clutter in different range intervals may come in at different times. Since the reflections from each type of clutter are received in time sequence the weight W of the loop must be set to zero and a new weight developed for each type of clutter. Since the weight developed must start from zero it must build up very quickly and hence have a short time constant. Because the weight builds up quickly to allow clutter cancellation, it also builds up quickly for fast moving desired targets and may also cancel them.